The undesirable effects of over exposure to sunlight are well known. Such exposure can result in not only uncomfortable sunburn but in prematurely aging skin, wrinkles, loss of skin elasticity, dermatosis, and skin cancer. Sunscreening is desirable in order to protect the skin from these, and other, adverse effects of solar radiation. The most dangerous solar radiation is the ultraviolet (UV) radiation at wave lengths lower than 400 nm which includes both UV-A and UV-B radiation.
Conventional sunscreen compositions are typically in the form of a liquid, either a lotion or a cream. These compositions may be either oil or water based. The water based emulsion serves mainly as an aid to disperse the active ingredients topically. The carrier water evaporates and leaves a thin film of active ingredients plus excipients deposited on the skin. The film remaining on the skin contains the product which protects the skin from ultraviolet radiation. Sunscreen compositions are typically rated by their sun protection factor (SPF) which is a measure of the protection from the sun afforded by the sunscreen agent or composition containing the sunscreen agent. Compositions having higher SPF values provide more protection from solar radiation.
Numerous disclosures of sunscreen compositions are available. For example, see U.S. Patent Publication No. US 2004/0126339 to Roszell, U.S. Pat. No. 5,518,712 to Stewart, U.S. Pat. No. 5,916,541 to Stewart, International Publication No. WO 97/42933 to Stewart, U.S. Pat. No. 6,074,630 to Devillez et al., U.S. Pat. No. 5,885,557 to Lentini, and U.S. Pat. No. 6,436,376 to Hansenne et al.
Sunscreen agents that primarily filter or absorb UV-A light are often referred to as UV-A absorbers. Similarly, sunscreen agents that primarily filter or absorb UV-B light are often referred to as UV-B absorbers. In general, the UV-A light refers to ultraviolet light having a wave length of 320-400 nm and UV-B light refers to ultraviolet light have a wave length of 280-320 nm.
UV-A absorbers have a tendency to degrade when exposed to light. For example, the UV-A absorber avobenzone (available under the name Parsol 1789) is known to degrade when exposed to light. See http://en.wikipedia.org/wiki/Parsol—1789 (avobenzone). A variety of ingredients have been proposed to enhance the photo stability of avobenzone, including Croda's new Optisol, and Ciba's Tinosorb S (Photochemistry and Photobiology, 2001, 74(3): 401-406). The photo stability of avobenzone remains highly formulation dependent. See http://www.cosmeticsdatabase.com/special/sunscreens/summary.php.